Filter member producing method

ABSTRACT

A method for fabricating a filtering member in which overlapping portions of a wire are bonded together in a layered manner through thermal treatment for forming a mesh is disclosed. In accordance with the method, a contact surface pressure between portions of the wire to be bonded together is maintained as equal to or higher than a predetermined level set depending on a thermal treatment condition. In this state, the thermal treatment is conducted such that a bonding portion of the wire has a strength equal to or greater than 4N.

FIELD OF THE INVENTION

The present invention relates to fabrication methods for a filteringmember in which a wire arranged in a layered manner is bonded integrallyfor forming a mesh, such as a coil type filter, which is, for example, afilter used in an airbag inflator.

BACKGROUND OF THE INVENTION

Conventionally, a vehicle is provided with an airbag device, whichinflates a bag by quickly supplying gas when a rapid deceleration occursdue to, for example, a car crash. The airbag device includes an inflatorfor quickly supplying gas when the device is activated and a baginflated by the gas from the inflator for protecting passengers. Theinflator incorporates an igniter, a gas generating agent burned in anexplosive-like manner by the heat generated by the igniter, and afilter. The filter collects and cools solid or liquid residue containedin hot gas generated through combustion of the gas generating agent. Thefilter is configured normally by a coil type filter, which is formed bywinding a circular wire or a deformed wire, such as a square wire,formed of metal (hereinafter, referred to as “a wire”) in a layeredmanner for defining a meshed cylindrical body. When gas or liquid passesthrough the mesh defined by the wire, the substance is cooled andfiltered by the coil type filter such that a residue is recovered.

After winding the wire of this type of filter, overlapping portions ofthe wire are bonded together by, normally, thermal treatment(sintering). In this manner, the filtration performance of the filter ismaintained by preventing the mesh from being deformed due to expansionof the wire or an impact caused by the gas passing through the mesh.

Particularly, a filter serving as a filtering member for an airbag isexposed to extremely hot gas. It is thus necessary to employ a bondingmethod that ensures a relatively high bonding strength. Therefore, forimproving the bonding strength, it has been proposed that the sinteringbe conducted at a higher temperature, or for a prolonged time, or in amodified atmosphere (see Japanese Laid-Open Patent Publication No.2001-171472).

However, in accordance with the fabrication method for a filter for anairbag inflator described in the aforementioned publication, it isnecessary to modify the corresponding equipment for obtaining a highersintering temperature or changing the sintering atmosphere. Suchmodification requires a relatively large cost, thus increasing themanufacturing cost. Further, the prolonged processing time decreasesproductivity.

Accordingly, in order to solve the aforementioned problems of the priorart, it is an objective of the present invention to provide afabrication method for a filtering member by which the bonding strengthof a wire forming a mesh is improved with relatively low cost andrelatively high efficiency.

SUMMARY OF THE INVENTION

In order to solve the aforementioned problem, an embodiment of thepresent invention provides a method for fabricating a filtering memberin which overlapping portions of a wire are bonded together in a layeredmanner through thermal treatment for forming a mesh. In accordance withthe method, a contact surface pressure between portions of the wire tobe bonded together is maintained as equal to or higher than apredetermined level set depending on a thermal treatment condition. Inthis state, the thermal treatment is conducted such that a bondingportion of the wire has a strength equal to or greater than 4 N (4Newtons).

It is desirable that, when a thermal treatment temperature and a thermaltreatment time are specified as the thermal treatment condition, thethermal treatment is performed such that the following inequality issatisfied:4≦C1× exp(−C2/T)×(t/T)^(0.4) ×P×b ² ×n

T: thermal treatment temperature, t: thermal treatment time, P: contactsurface pressure, b: lateral contact dimension between contact portionsof the wire, n: number of bonding portions of the wire;

C1, C2 are coefficients; C1=4,105, C2=9,000.

It is desirable that the filtering member be a coil type filter in whichthe wire is wound in a layered manner for forming a mesh and that thecontact surface pressure be reliably produced by tension applied to thewire during winding of the wire.

It is desirable that a winding end of the wire be fixed whilemaintaining the tension applied to the wire during winding of the wirein a state acting on the wire. It is also desirable that the contactsurface pressure be adjusted by changing the tension applied to the wireduring winding of the wire.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing an inflator;

FIG. 2(a) is a perspective view showing a filter;

FIG. 2(b) is an enlarged view showing a portion of the filter;

FIG. 3 is a graph showing variation of the pressure generated in achamber as time elapses; and

FIG. 4 is a graph showing contact surface pressure versus bondingstrength between bonding portions of a wire.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A fabrication method for an airbag inflator filter (a filtering member)incorporated in an inflator of an airbag device according to oneembodiment of the present invention will now be described with referenceto FIGS. 1 to 4.

As shown in FIG. 1, in the illustrated embodiment, a detonator 11 and aburnable igniter 12 are installed in a middle portion of an inflator 10of the airbag device. The detonator 11 is activated in response to anactivation signal from a sensor (not shown). A chamber portion 13 isdefined around the outer circumference of the detonator 11 and that ofthe igniter 12. The chamber portion 13 receives the heat generatedthrough activation of the detonator 11 and resulting combustion of theigniter 12. A gas generating agent 14 is installed in the chamberportion 13. The gas generating agent 14 generates a relatively largeamount of gas when burned in an explosion-like manner by the heatgenerated through the activation of the detonator 11 and the combustionof the igniter 12. The generated gas is then supplied to a bag (notshown) of the airbag device.

A filter 15 is arranged in the inflator 10 and encompasses the chamberportion 13. The filter 15 functions as a cooling member for cooling thehot gas generated through combustion of the gas generating agent 14 andsupplying the gas and as a filtering member for recovering solid residuecontained in the gas.

With reference to FIGS. 2(a) and 2(b), in order to fabricate the filter15, a circular wire or a deformed wire, such as a square wire, formed ofmetal (hereinafter, referred to as “a wire”) 16 is wound around acylindrical bobbin (not shown) for defining a mesh. The bobbin is thenremoved such that the filter 15 is shaped like a hollow cylinder. In thefilter 15 of the illustrated embodiment, a wire material formed mainlyof iron (having a cross-sectional area of 0.2 mm²) is employed as thewire 16, The wire 16 is wound around the outer circumferential surfaceof the bobbin for 500 cycles, thus defining a mesh. The filter 15 has ahollow cylindrical shape with an outer diameter of φ60 mm and an innerdiameter of φ50 mm.

Accordingly, when a relatively large amount of hot gas passes throughthe gaps of the mesh formed by the wire 16, the filter 15 is allowed tocool the gas and recover solid residue contained in the gas. Further, inthe illustrated embodiment, the winding pitch of the wire 16 is definedas a pitch C, the angle between the crossing portions of the wire 16 isdefined as a crossing angle θ, the dimension of the wire 16 in thebobbin axial direction is defined as a winding dimension L, and thecrossing portions of the wire 16 are referred to as a contact portion S,referring to FIGS. 2(a) and 2(b).

The fabrication method for the filter 15 will be explained in detail asfollows. First, the wire 16 is wound around the outer circumferentialsurface of the bobbin in a crisscross manner, with a predetermined levelof tension applied to the wire 16, such that a mesh is formed by thewire 16. The wire 16 may be wound around the bobbin by moving the wire16 in the axial direction of the bobbin or moving the shaft of thebobbin in the bobbin's axial direction. Regarding the filter 15, thepitch C, the winding dimension L, the crossing angle θ of the wire 16are obtained through computer simulation. The filter 15 is thusfabricated in accordance with the optimal values of these parameters. Inthis manner, the mesh pattern or the winding density are set as desired,such that various types of meshes may be formed depending on differentrequirements for the filtering function.

When winding of the wire 16 is completed, a winding end 17 of the wire16 is joined (bonded) with an adjacent winding portion through weldingor the like, with the tension continuously applied to the wire 16. Thebobbin is then removed from the wire 16, thus producing an unfinished,hollow cylindrical filter that has yet to be thermally treated.Subsequently, for bonding the contact portions S of the wire 16, thewire 16 is thermally treated and sintered, such that a coil type filter15 shaped as shown in FIG. 2(a) is completed.

If the filter 15 is used as a filtering member for an airbag, the filter15 is exposed to extremely hot gas (at approximately 2,200 degreesCelsius). It is thus necessary to bond the contact portions S togetherwith a relatively high bonding strength. Accordingly, for meeting suchnecessity, certain conditions are set in fabrication of the filter 15 inaccordance with the illustrated embodiment. These conditions will beexplained as follows.

FIG. 3 is a graph representing the pressure variation in the chamberportion 13 in the inflator 10 as time elapses when the airbag isactuated. The graph of FIG. 3 indicates that the pressure in the chamberportion 13 reaches a maximum level, approximately 4 MPa, immediatelyafter actuation of the airbag. Since the filter 15 is arranged in thechamber portion 13, the bonding strength of the contact portions S ofthe filter 15 must be sufficiently high for tolerating the pressure ofapproximately 4 MPa.

By the following equation (1), a tension σ applied to a single sectionof a wire of a coil type filter formed by winding a wire such as a metalline around the outer circumferential surface of a cylindrical bobbin,like the filter 15, is determined:σ=γ₂ ² P1/(γ₁ ²/γ₂ ²)×(γ₁ ²/γ₂ ²+1)×Z  (1)

In this equation, a represents the tension applied to the single wiresection, P1 represents the interior pressure acting on the filter, γ₁ isthe outer diameter of the filter, γ₂ is the inner diameter of thefilter, γ is the radius of the filter (γ₂≦γ≦γ₁), and Z represents thecross-sectional area of the wire.

It can be considered that the equation (1) represents the tensionapplied to a single section of the wire 16 when the gas generating agent14 is burned. Thus, by applying the values representing thespecification of the filter 15 (the wire cross-sectional area: 0.2 mm²,the number of winding cycles: 500, the outer diameter: (φ60, the innerdiameter: (φ50) to the equation (1), a value a substantially equal to 4N is obtained. It is thus understood that a load of about 4 N acts onthe single section of the wire 16 of the filter 15 as the tension, whenthe airbag is actuated.

Therefore, in order to prevent the bonding of the different sections ofthe wire 16 from loosening when the airbag is actuated, the bondingstrength must be sufficiently high for producing resistance against aload of about 4 N. In other words, it is required that sintering beperformed while the contact surface pressure of each of the contactportions S is maintained as equal to or higher than a predeterminedlevel corresponding to sintering conditions, such that the bondingstrength of the bonding portions of the wire 16, which defines the mesh,is equal to or higher than 4 N.

As is broadly known, sintering of a metal line, which corresponds to thewire 16, is brought about by diffusion of atoms configuring the metalline toward a bonding point. The following equation (2) representingsintering through internal diffusion is thus selected:x ⁵ /a ²=10γVD _(v) t/RT  (2)

In this equation, x represents the radius of a substantially circularcontact surface formed through sintering of the metal line or a half ofthe lateral dimension of a strip-like contact surface if the contactsurface obtained through sintering is developed in a strip-like shape, arepresents the radius of the metal line, γ represents the surfacetension of the metal line, V represents the volume of 1 mol of the metalline, D_(V) represents the diffusion coefficient in sintering, trepresents the sintering time, R represents the gas constant (1.987cal/degree), and T represents the sintering temperature (the absolutetemperature). The equation (2) was obtained with reference to pages 138to 141 of “General Remarks on Powder Metallurgy Sintering Mechanism”(published by Nikkan Kogyo Shinbunsha, Mar. 24, 1964).

Further, by conducting tests, the following equation (3) related to thebonding strength F, the sintering temperature T, the sintering time t,the contact surface pressure P between the bonded sections of the wire16, the lateral contact dimension between the contact portions S of thewire 16, and the number n of the bonding portions of the wire 16 wasobtained:F=C1 exp (−C2/T)×(t/T)^(0.4) ×P×b ² ×n  (3)

In this equation, C1 and C2 each represent a coefficient and satisfy thefollowing: C1=4105, C2=9000.

The coefficients C1, C2 are determined based on the assumption that thesquared value of x (the radius of the contact surface and the like) ofthe equation (2) corresponds to the bonding area (or the bondingstrength) brought about through sintering of the contact portions S ofthe filter 15, in accordance with the graph of FIG. 4 representing therelationship between the bonding strength between the bonding portionsof the wire 16 and the contact surface pressure between the bondingportions of the wire 16.

The graph of FIG. 4 shows the results of the test that has beenconducted for determining the relationship between the contact surfacepressure and the bonding strength between the bonding portions of thewire 16, representing the relationship between the bonding strengthbetween the bonding portions of the wire 16 and the contact surfacepressure between the bonding portions of the wire 16 in accordance withsintering conditions (temperature×time) when the number n of the bondingportion of the wire 16 corresponds to one. More specifically, in thegraph, the relationship between the bonding strength between the bondingportions of the wire 16 and the contact surface pressure between thebonding portions of the wire 16 in accordance with a first sinteringcondition (1,100 degrees Celsius×30 minutes) is represented by the solidline (a) and such relationship in accordance with a second sinteringcondition (1,100 degrees Celsius×10 minutes) is represented by the solidline (b). Further, the relationship between the bonding strength betweenthe bonding portions of the wire 16 and the contact surface pressurebetween the bonding portions of the wire 16 in accordance with a thirdsintering condition (1,000 degrees Celsius×30 minutes) is represented bythe solid line (c) and such relationship in accordance with a secondsintering condition (1,000 degrees Celsius×10 minutes) is represented bythe solid line (d).

Since the bonding strength F between the bonding portions of the wire 16of the filter 15 must be equal to or higher than 4 N for reaching asufficient level, the filter 15 should be fabricated such that thecondition represented by the following equation (4), which is obtainedfrom the test equation (3), is satisfied:4≦C1× exp(−C2/T)×(t/T)^(0.4) ×P×b ² ×n  (4)

Regarding the equation (4), T, t, P, b, n, C1, and C2 are defined in thesame manner as those of the equation (3).

Based on the equation (4) and FIG. 4, the following is understood. Thatis, in the case of the first sintering condition (1,100 degreesCelsius×30 minutes) of FIG. 4, a filter 15 having a bonding strength Fequal to or higher than 4 N is obtained by setting the contact surfacepressure to 0.25 N/mm² or higher. If the contact surface pressure is setto 0.39 N/mm² or higher, a filter 15 having a bonding strength F equalto or higher than 4 N can be obtained while improving productivity, asin the case of the second sintering condition (1,100 degrees Celsius×10minutes). Likewise, if the contact surface pressure is set to 0.39 N/mm²or higher, a filter 15 having a bonding strength F equal to or higherthan 4 N can be obtained at a sintering temperature lower than that ofthe first sintering condition, as in the case of the third sinteringcondition (1,000 degrees Celsius×30 minutes).

Further, if the contact surface pressure is set to 0.62 N/mm² or higher,a filter 15 having a bonding strength F equal to or higher than 4 N canbe obtained at a relatively low sintering temperature with improvedproductivity, as in the case of the fourth sintering condition (1,000degrees Celsius×10 minutes).

Accordingly, in a filter 15 in which the overlapping portions (thecontact portions S) of the wire 16 are bonded together through sinteringin a layered manner for defining a mesh, it is indicated that the levelof the surface pressure (the contact surface pressure) between thebonding portions of the wire 16, as well as the sintering conditionsincluding the sintering temperature, is an important factor fordetermining the bonding strength. For applying the contact surfacepressure to each of the contact portions S, a tapered jig may beinserted into the hollow shaft portion of the filter 15 when sinteringis performed such that the crossing portions of the wire 16 are held incontact as pressed together. However, in a coil type filter formed bywinding the wire 16 around a bobbin, such as the filter 15 of theillustrated embodiment, the contact surface pressure can be produced bythe tension applied to the wire 16 during winding. The contact surfacepressure is thus ensured relatively easily.

The following equation (5) represents the relationship between thecontact surface pressure between the bonding portions of the wire 16 andthe tension in the case of the illustrated embodiment.P=σ/(r×b)  (5)

In the equation (5), P represents the contact surface pressure (N/mm²)between the wire bonding portions, σ represents the tension (N) actingon a single section of the wire 16, r represents the winding radius (mm)of the wire 16, and b represents the lateral contact dimension (mm)between the bonding portions of the wire 16.

As is understood from the equation (5), if a coil type filter 15 isfabricated with tension applied to the wire 16, the tension applied tothe wire 16 during winding is a factor related to the contact surfacepressure of the contact portions S. In other words, as the tension σapplied during winding becomes larger, the contact surface pressure Pbecomes higher. In contrast, as the tension σ applied during windingbecomes smaller, the contact surface pressure P becomes lower. Forexample, if it is required that the contact surface pressure between thebonding portions of the wire 16 is set to 0.62 N/mm² or higher, thetension σ obtained by the equation (5) is 9.61 N when the lateralcontact dimension b is 0.5 mm and the winding radius r is 31 mm.However, since the actual process temperature is extremely high, thetension σ as a whole cannot act as the contact surface pressure. Also,the actual number n of the bonding portions of the wire 16 cannot beone. With these conditions considered, it is known that the actuallyobtained effect corresponds to at least 60% of the computed value. Thus,by setting the tension a applied to the wire 16 during winding to 16 Nor larger, the contact surface pressure at each of the contact portionsS of the wire 16 can be set to 0.62 N/mm² or higher.

Further, if the tension applied to the wire 16 during winding of thewire 16 is changed in accordance with the equation (5) when the filter15 is fabricated as a filtering member, it is indicated that the contactsurface pressure, which is an important factor for determining thebonding strength, is adjusted. The filter 15 having a different bondingstrength is thus obtained. Also, such contact surface pressure caused incorrespondence with the tension applied to the wire 16 during winding ismaintained by fixing the winding end 17 of the wire 16 to a differentportion of the filter 15 (for example, an intermediate section of thewire 16) through welding or swaging, with the tension applied to thewire 16 during winding maintained in a non-released state.

The illustrated embodiment has the following advantages.

(1) In the illustrated embodiment, for setting the bonding strength ofthe bonding portions of the wire 16, which defines a mesh, to 4 N orgreater, sintering is performed with the contact surface pressurebetween the bonding portions of the wire 16, or at each of the contactportions S, maintained as equal to or higher than a predetermined levelset depending on the sintering conditions. Thus, regardless of therelatively high pressure and high temperature caused by actuation of theairbag, the bonding portions of the wire 16 at the contact portions Sare maintained without loosening. Accordingly, a relatively high bondingstrength is ensured in the filter 15 with relatively low cost andimproved efficiency.

(2) In the fabrication method of the filter 15 of the illustratedembodiment, regarding the sintering process, the relationship amongnumerals including the sintering temperature, the sintering time, thecontact surface pressure between the bonding portions of the wire 16,the lateral contact dimension between the bonding portions of the wire16, and the number of the bonding portions of the wire 16 is set suchthat the condition of the predetermined equation (4) is satisfied. Inother words, in order to set the bonding strength between the bondingportions of the wire 16 to a value equal to or higher than a certainvalue (4 N), the filter 15 is fabricated in such a manner as to satisfythe condition represented by the equation (4), which represents therelationship among the sintering temperature and sintering time of thefilter 15, the contact surface pressure between the bonding portions ofthe wire 16, the lateral contact dimension between the bonding portionsof the wire 16, and the number of the bonding portions of the wire 16.Accordingly, the filter 15 is configured to reliably tolerate the gaspressure produced by actuation of the airbag. Further, by optimizing thevarious conditions (T: sintering temperature, t: sintering time, P:contact surface pressure between the wire bonding portions, b: lateralcontact dimension between the wire bonding portions, and n: number ofthe wire bonding portions), the process conditions that are suitable forthe performance of production equipment and capable of maximallyimproving productivity can be selected for fabricating the filter 15.

(3) In the illustrated embodiment, tension is applied to the wire 16when the wire 16 is wound. A sufficient contact surface pressure is thusensured at each of the contact portions S at which the correspondingsections of the wire 16 cross each other, when the filter 15 isfabricated. That is, in fabrication of the filter 15, the necessarycontact surface pressure, which is an important factor for determiningthe bonding strength between the bonding portions of the wire 16, isobtained easily and reliably.

(4) In the illustrated embodiment, the winding end 17 of the wire 16 isfixed (bonded) through welding or the like with the tension maintainedas applied to the wire 16, in fabrication of the filter 15. Suchfabrication is thus completed with the contact surface pressuremaintained at a required sufficient level during sintering.

(5) When a filter 15 is fabricated as the filtering member in theillustrated embodiment, the tension applied to the wire 16 duringwinding is changed in correspondence with the predetermined equation(5), such that the contact surface pressure is adjusted. It is thuspossible to easily optimize the contact surface pressure, which isvaried in accordance with changes in the sintering conditions.

The illustrated embodiment may be modified as follows.

In the illustrated embodiment, a line formed mainly of iron (having across-sectional area of 0.2 mm²) is employed as the wire 16. The wire 16is wound around the bobbin for 500 cycles for forming a hollowcylindrical coil type filter 15 with an outer diameter of φ60 and aninner diameter of φ50. However, the material or the dimensions of thefilter 15 may be selected as needed in accordance with the shape or thedimensions of the inflator 10, in which the filter 15 is installed.

In the filter 15 of the illustrated embodiment, the mesh is formed bywinding the wire 16, the metal square or circular line, around the shaftof the bobbin. However, the mesh of the filter 15 may be formed indifferent suitable manners modified from that of the embodiment. Forexample, the filter 15 may include a knitted mesh such as a stockinettype mesh. Alternatively, the mesh of the filter 15 may be formed bywinding a flat-woven mesh in an overlapping manner.

In the illustrated embodiment, the contact surface pressure acting oneach of the contact portions S between the bonded portions of the wire16 of the filter 15 is produced by the tension applied to the wire 16during winding of the wire 16. However, such contact surface pressuremay be generated using a tapered jig or the like, when sintering isconducted.

In the illustrated embodiment, the overlapping portions of the wire 16,which are arranged in a layered manner, are bonded together thoughsintering. However, bonding of the wire 16 may be brought about bydifferent suitable methods other than the sintering. For example, theoverlapping portions of the wire 16 may be bonded together throughhigh-frequency induction heating.

1. A method for fabricating a filtering member in which overlappingportions of a wire are bonded together in a layered manner throughthermal treatment for forming a mesh, the method comprising: applying acontact surface pressure between portions of the wire to be bondedtogether; and maintaining the contact surface pressure as equal to orhigher than a predetermined level that is set in accordance with athermal treatment condition, and conducting the thermal treatment inthis state, such that each bonding portion of the wire has a strengthequal to or greater than 4 N.
 2. The method according to claim 1,wherein, when a thermal treatment temperature and a thermal treatmenttime are specified as the thermal treatment condition, the thermaltreatment is performed such that the following inequality is satisfied:4≦C1× exp(−C2/T)×(t/T)^(0.4) ×P×b ² ×n in which: T: thermal treatmenttemperature, t: thermal treatment time, P: contact surface pressure, b:lateral contact dimension between contact portions of the wire, n:number of bonding portions of the wire, and wherein C1 and C2 arecoefficients, with C1=4,105, and C2=9,000.
 3. The method according toclaim 1, wherein: the filtering member is a coil type filter in whichthe wire is wound in a layered manner for forming a mesh, and thecontact surface pressure is produced by tension applied to the wireduring winding of the wire.
 4. The method according to claim 3, whereina winding end of the wire is fixed while the tension is applied to thewire during winding of the wire.
 5. The method according to claim 3,wherein the contact surface pressure is adjusted by changing the tensionapplied to the wire during winding of the wire.
 6. A method forfabricating a filter for an airbag inflator in which overlappingportions of a metal wire are bonded together in a layered manner throughthermal treatment for forming a mesh, the method comprising: applying acontact surface pressure between portions of the wire to be bondedtogether; and maintaining the contact pressure as equal to or higherthan a predetermined level that is set in accordance with a thermaltreatment condition, and conducting the thermal treatment in this state,such that each bonding portion of the wire has a strength equal to orgreater than 4 N.
 7. The method according to claim 6, wherein, when athermal treatment temperature and a thermal treatment time are specifiedas the thermal treatment condition, the thermal treatment is performedsuch that the following inequality is satisfied:4≦C1× exp(−C2/T)×(t/T)^(0.4) ×P×b ² ×n in which: T: thermal treatmenttemperature, t: thermal treatment time, P: contact surface pressure, b:lateral contact dimension between contact portions of the wire, n:number of bonding portions of the wire, and C1 and C2 are coefficients,with C1=4,105, and C2=9,000.
 8. The method according to claim 6, whereinthe filter is a coil type filter in which the wire is wound in a layeredmanner for forming a mesh, and the contact surface pressure is producedby tension applied to the wire during winding of the wire.
 9. The methodaccording to claim 8, wherein a winding end of the wire is fixed whilethe tension is applied to the wire during winding of the wire.
 10. Themethod according to claim 8, wherein the contact surface pressure isadjusted by changing the tension applied to the wire during winding ofthe wire.